[TeZla]

There's a start.

In a simple explanation, an alternator can be turned into a 3 phase AC motor. You need a suitable controller to do that. you can get them off ebay for a good price, Just look for something suited to an ebike or similar sized motor. As it stands though, they don't make very good motors, as there is no way for the controller to 'know' where the rotor is in its phase cycle. you can fix that by installing 3 hall sensors into the stator, to detect the rotor.
That is a good little project that'll teach you about 3 phase motor basics, but there is still a bit of a learning curve in figuring out where to put the hall sensors, how to wire them, what order they go in etc.
You also need to feed power to the rotor, via the slip rings and a modified/gutted regulator. Its not as simple as just feeding it 12v either, while that will work, there is a lot of tuning to get the voltage just right, which also varies depending on the rotor speed.

If your not afraid of a bit of tinkering, circuitry and theory, then jump right into it. If your expecting it to be a really simple hand held excersize, probably stop now and just buy a proper designed motor.